TY - JOUR
T1 - Rationalizing an Unexpected Structure Sensitivity
in Heterogeneous Catalysis—CO Hydrogenation over Rh as a Case
Study
AU - Schumann, Max
AU - Nielsen, Monia R.
AU - Smitshuysen, Thomas E. L.
AU - Hansen, Thomas W.
AU - Damsgaard, Christian D.
AU - Yang, An-Chih A.
AU - Cargnello, Matteo
AU - Grunwaldt, Jan-Dierk
AU - Jensen, Anker D.
AU - Christensen, Jakob M.
PY - 2021
Y1 - 2021
N2 - A common expectation in heterogeneous catalysis is that the optimal
activity will occur for the particle size with the highest concentration
of undercoordinated step, edge, or corner sites, expectedly in the
<5 nm range. However, many metal-catalyzed reactions follow a
different trend, where the turnover frequency (TOF, here rate per
surface atom) is instead lower for these smaller particles and increases
strongly with increasing size toward a stabilized level with a
size-independent TOF. Here, we use one of these reactions, the
Rh-catalyzed CO hydrogenation to hydrocarbons and C2-oxygenates, to illuminate the origin of this effect. Studying Rh/SiO2
catalysts, we show that smaller (<4 nm) Rh particles are richer in
undercoordinated edge, corner, and step sites, but are nevertheless of
lower activity because the entire surface, including the planar facets,
is shifted to a prohibitively high adsorbate coverage—in this case of
CO. In transient experiments, where the inhibiting adsorbates are
allowed to desorb, smaller 1.7 nm Rh particles and larger 3.7 nm Rh
particles reach similar rates of CO activation despite the steady-state
TOF being an order of magnitude higher on the larger particles. This
shows that it is a prohibitive adsorbate coverage under reaction
conditions rather than a lower number of active sites or a lower
intrinsic activity of the sites that causes the lower activity of the
smaller particles. In steady-state experiments at 20 bar, the TOF for CO
hydrogenation increases by 55% from 3.7 nm Rh particles to 5.3 nm Rh
particles even though the measured concentration of step sites decreases
by 30% in this size range. This indicates that such undercoordinated
sites are not necessarily the primary active centers and that the
reaction is instead focused on the planar facets. The reaction kinetics
show that the reaction becomes increasingly pressure-dependent with
increasing particle size, implying that the surface becomes increasingly
free of adsorbates on larger particles. Taken together with the
indications that the reaction may be focused on the planar facets, this
leads to the new insight that it is a prohibitively high adsorbate
coverage on the entire surface (and not just on a minority of
undercoordinated sites) that is the primary reason for the low activity
of small nanoparticles. The identification of a detrimental
high-coverage state for small particles is expected to be of general
relevance to the many industrially important reactions sharing the same
behavior. The high-coverage state is not exclusively negative, but can
also facilitate different reaction pathways. It is the higher CO
coverage on small particles that drives the C2-oxygenate
formation and is the reason for the high selectivity of rhodium to such
complex products, which is at its highest for the smallest (∼2 nm)
investigated particles.
AB - A common expectation in heterogeneous catalysis is that the optimal
activity will occur for the particle size with the highest concentration
of undercoordinated step, edge, or corner sites, expectedly in the
<5 nm range. However, many metal-catalyzed reactions follow a
different trend, where the turnover frequency (TOF, here rate per
surface atom) is instead lower for these smaller particles and increases
strongly with increasing size toward a stabilized level with a
size-independent TOF. Here, we use one of these reactions, the
Rh-catalyzed CO hydrogenation to hydrocarbons and C2-oxygenates, to illuminate the origin of this effect. Studying Rh/SiO2
catalysts, we show that smaller (<4 nm) Rh particles are richer in
undercoordinated edge, corner, and step sites, but are nevertheless of
lower activity because the entire surface, including the planar facets,
is shifted to a prohibitively high adsorbate coverage—in this case of
CO. In transient experiments, where the inhibiting adsorbates are
allowed to desorb, smaller 1.7 nm Rh particles and larger 3.7 nm Rh
particles reach similar rates of CO activation despite the steady-state
TOF being an order of magnitude higher on the larger particles. This
shows that it is a prohibitive adsorbate coverage under reaction
conditions rather than a lower number of active sites or a lower
intrinsic activity of the sites that causes the lower activity of the
smaller particles. In steady-state experiments at 20 bar, the TOF for CO
hydrogenation increases by 55% from 3.7 nm Rh particles to 5.3 nm Rh
particles even though the measured concentration of step sites decreases
by 30% in this size range. This indicates that such undercoordinated
sites are not necessarily the primary active centers and that the
reaction is instead focused on the planar facets. The reaction kinetics
show that the reaction becomes increasingly pressure-dependent with
increasing particle size, implying that the surface becomes increasingly
free of adsorbates on larger particles. Taken together with the
indications that the reaction may be focused on the planar facets, this
leads to the new insight that it is a prohibitively high adsorbate
coverage on the entire surface (and not just on a minority of
undercoordinated sites) that is the primary reason for the low activity
of small nanoparticles. The identification of a detrimental
high-coverage state for small particles is expected to be of general
relevance to the many industrially important reactions sharing the same
behavior. The high-coverage state is not exclusively negative, but can
also facilitate different reaction pathways. It is the higher CO
coverage on small particles that drives the C2-oxygenate
formation and is the reason for the high selectivity of rhodium to such
complex products, which is at its highest for the smallest (∼2 nm)
investigated particles.
U2 - 10.1021/acscatal.0c05002
DO - 10.1021/acscatal.0c05002
M3 - Journal article
VL - 11
SP - 5189
EP - 5201
JO - ACS Catalysis
JF - ACS Catalysis
SN - 2155-5435
ER -